WIP: Add cop for DiscourseEvent.on

config/default.yml

@@ -67,3 +67,9 @@ Discourse/Plugins/CallRequiresPlugin:
   Enabled: true
   Include:
     - 'app/controllers/**/*'
+
+Discourse/Plugins/DiscourseEvent:
+  Enabled: true
+  Include:
+    - 'plugin.rb'
+    - 'plugins/*/plugin.rb'

lib/rubocop/cop/discourse/plugins/discourse_event.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Discourse
+      module Plugins
+        class DiscourseEvent < Base
+          MSG =
+            "Use `on` instead of `DiscourseEvent.on` as the latter will listen to events even if the plugin is disabled."
+          RESTRICT_ON_SEND = [:on].freeze
+
+          def_node_matcher :discourse_event_on?, <<~MATCHER
+            (send (const nil? :DiscourseEvent) :on _)
+          MATCHER
+
+          def on_send(node)
+            return unless discourse_event_on?(node)
+            add_offense(node, message: MSG)
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/rubocop/cop/plugins/discourse_event_spec.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe RuboCop::Cop::Discourse::Plugins::DiscourseEvent, :config do
+  subject(:cop) { described_class.new(config) }
+
+  let(:config) { RuboCop::Config.new }
+
+  context "when `DiscourseEvent.on` is called" do
+    it "registers an offense" do
+      expect_offense(<<~RUBY)
+        DiscourseEvent.on(:topic_status_updated) { do_something }
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Discourse/Plugins/DiscourseEvent: Use `on` instead of `DiscourseEvent.on` [...]
+      RUBY
+    end
+  end
+
+  context "when `on` is called" do
+    it "does not register an offense" do
+      expect_no_offenses(<<~RUBY)
+        on(:topic_status_updated) { do_something }
+      RUBY
+    end
+  end
+end